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Octavius (Welsh: Eydaf) was a legendary king of the Britons in Geoffrey of Monmouth's Historia regum Britanniae, a fictional account of the rulers of Britain. He reigned when Constantine I was emperor in Rome. Constantine had been crowned king of the Britons upon his father's death in 306 at York. As Constantine became more powerful, he was forced to leave Britain in the hands of a proconsul while he ruled in Rome. This prompted Octavius, then Duke of the Gewissei tribe of Celts, to begin a rebellion against Roman rule. Octavius murdered the proconsul and his aides then took the crown of Britain upon his own head.

This led to a response from Constantine who sent three legions to Britain under the command of his great-uncle, Trahern, the brother of the late King Coel. He landed at Kaerperis and captured it forcing Octavius to conscript all the island into the army to combat the Roman legions. Octavius met Trahern in a field outside Winchester where they fought and Octavius won. Trahern fled to Albany and pillaged the land. That caused Octavius to react again, where he met Trahern in Westmorland, but Octavius was defeated this time. Octavius fled Britain and Trahern took on the crown himself.

Octavius fled to Norway where he sought help from King Gunbert. In Britain, supporters of Octavius managed to ambush Trahern and kill him near London, allowing Octavius to return to Britain. Once arrived, he scattered the Roman forces and retook the throne of Britain, gaining an incredible amount of wealth and prestige. Following years of rule, Octavius decided to marry his only daughter off to the Roman emperor Maximianus. After a brief struggle with his nephew, Conan Meriadoc, the kingship of Britain was given to Maximianus, and Octavius abdicated.